Amyloid deposits found in neurodegenerative diseases result from misfolding of cellular proteins. The challenge for developing specific inhibitors that block oligomer or fibril formation is that there are no high- resolution molecular structures that can guide the design. The proposal has three specific aims. The first aim is to use high-resolution solid-state NMR in combination with atomic force microscopy (AFM) to refine the structures that are emerging of Abeta oligomers and fibrils. The second aim is to design inhibitors based on the structural data from Aim 1 and to test their ability in vitro to block oligomer or fibril formation. The third aim is to assay the ability of these inhibitors to block the toxicity of Abeta oligomers and fibrils on neuronal cells. A new approach has been developed for obtaining high-resolution AFM images of Abeta soluble oligomers and fibrils in solution. The method takes advantage of a novel AFM controller that provides resolution in 'single touch'AFM experiments that surpasses the resolution currently available using commercial instruments. High resolution AFM of solution samples will allow us to follow the formation of Abeta oligomers, protofibrils and fibrils, and determine how designed inhibitors prevent fibrillization. We have developed structural models for the Abeta42 monomer, dimer, protofibril and fibril based on preliminary results from high resolution AFM and solid-state NMR. The structures show that when the b-strands have a parallel orientation and the amino acids are in-register with one another, the surface of the b-sheet has pronounced ridges and grooves. This architecture provides the key elements for the rational design of inhibitors to prevent fibril formation. Our template inhibitor peptide based on a rational design approach has the sequence GxFxGxF, where the bulky phenylalanine side chains of the inhibitor are predicted to pack against the glycines in the GxxxG motif of the amyloidogenic peptide. We will test the ability of the designed peptides to disrupt the formation of oligomers and fibrils by thioflavin T fluorescence, size exclusion chromatography, electron microscopy, AFM and solid-state NMR. We will also test the ability of our designed inhibitors to protect neurons from cell death induced by amyloid fibrils. We will focus on the Abeta42 peptide because of its higher ability to form aggregates than the shorter isoforms. Moreover, most gene mutations that are associated with the inherited forms of Alzheimer's disease result in an increase in the ratio of Abeta42 over Abeta40.